Electro-mechanical oscillators may be used to drive electro-optical mechanisms such as printers, scanners, barcode readers and similar devices. A torsion oscillator, which is one type of oscillation device and may also be referred to as a resonant galvanometer, typically includes a mirror that is disposed on a plate that is cut or etched from a silicon wafer and supported on trunnions. In one embodiment, magnets are attached to the plate and when electric current passes through a nearby coil, a force is exerted on the magnets. This force is translated to the plate and causes oscillation of the plate which twists the trunnions. Other forces may be employed to make such a system oscillate, such as electric fields or mechanical forces. The plate is excited to oscillate, preferably at or near a resonant frequency, by an oscillation controller that causes current to pass through the coil at or near a drive or power level that results in the plate oscillating at or near the resonant frequency or at or near a harmonic of the resonant frequency.
The angle of the mirror moves sinusoidally with respect to time at a certain amount of sweep (termed amplitude), at a certain repetition rate (termed frequency), and with a potential lack of symmetry (termed median offset). The characteristics of mirrors can vary significantly due to physical variations from manufacturing tolerances and changing environmental conditions. Specifically, changes in air density affect drive efficiency, the resonant frequency of a torsion oscillator, and the ability to detect resonance. Thus, changes in elevation or other environmental parameters may affect the sweep performance of a torsion oscillator. The present invention is a method for controlling a torsion oscillator that reduces instances of off-resonant operation given changes in environmental conditions.
The above and other needs are met by a method that addresses the effects of air density on resonant frequency detection by using closed-loop feedback to ensure that the drive amplitude (Ad) at the open-loop drive frequency (fdOL=frOL−OS) is approximately equal to the closed-loop steady-state amplitude (ACL) and by using a closed-loop frequency sweep to determine the effect of air density on the open-loop resonant frequency sweep.
Each time the imaging system is reset (such as during power-on reset), information from a previous operation of the torsion oscillator is read from non-volatile memory. This information includes values which allow the imaging system to determine the most recent resonant frequency of the torsion oscillator, the last measured overshoot of the resonant frequency sweep algorithm and the open-loop drive level required to make the peak open-loop scan amplitude approximately equal to the closed-loop steady-state amplitude. The system first performs an open-loop resonant frequency sweep, applies the last measured overshoot to the result and drives the torsion oscillator at the determined drive frequency. If the measured amplitude is not within an acceptable amount of the desired closed-loop steady-state amplitude for the drive frequency, then the open-loop drive level is increased or decreased by a set amount or by a characterized input vs. output function, and the open-loop resonant frequency sweep is repeated until the drive amplitude (Ad) and closed-loop amplitudes (ACL) are acceptably close to each other. The imaging system now has an updated value of what open-loop drive level (POL) will produce approximately the same scan amplitude as what is desired during closed-loop steady-state operation.
Once the result of the open-loop resonant frequency sweep produces acceptable amplitude results, the imaging system enters closed-loop control. The imaging system then performs a closed-loop sweep of frequency and amplitude target pairs while monitoring the closed-loop controller feedback. The closed-loop steady-state resonant frequency is the frequency for which the controller output was a minimum. Alternatively, a conditional search for the minimum controller output can be performed, starting with the frequency and amplitude target on initial closed loop entry, and incrementing the target pairs in the direction indicating decreasing controller output until a minimum is found. The frequency corresponding to the minimum controller feedback is the closed-loop steady-state resonant frequency. The difference between the frequency at controller minimum and the resonant frequency detected by the open-loop sweep is the overshoot of the open-loop sweep. The imaging system now has an updated value of the overshoot and of the torsion oscillator closed-loop steady-state resonant frequency. The overshoot value is primarily a function of air density and will not change as long as the open-loop sweep method (e.g., rate at which the drive frequency is changed during the sweep) does not change, so the closed-loop sweep need not be repeated until the next POR.
The imaging system monitors the closed-loop controller feedback while the torsion oscillator is operating in steady-state. This feedback may be incorporated into the drive level of the next open-loop resonant frequency sweep to ensure that the open-loop amplitude will continue to be approximately equal to the closed-loop steady-state amplitude. Making use of the closed-loop feedback in this manner helps ensure that the next open-loop resonant frequency sweep will not fail the open-loop vs. closed-loop amplitude check, which would result in a longer time to first print due to the need to repeat the open-loop sweep.
The imaging system now has a record of the last detected resonant frequency, the open-loop drive level necessary to produce a valid resonant frequency sweep and the correct overshoot of the resonant frequency sweep. All subsequent resonant frequency sweeps should now complete in the shortest amount of time possible without sacrificing accuracy. The torsion oscillator will exhibit the desired scanning motion and will therefore not introduce print artifacts associated with off-resonant operation of the torsion oscillator.